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arXiv:1009.5733v1 [astro-ph.EP] 29 Sep 2010 The Lick-Carnegie Exoplanet Survey: A 3.1M?Planet in the

Habitable Zone of the Nearby M3V Star Gliese 581

Steven S. Vogt

1, R. Paul Butler2, E. J. Rivera1, N. Haghighipour3, Gregory W. Henry4,

and Michael H. Williamson 4

Received

; accepted

1UCO/Lick Observatory, University of California, Santa Cruz, CA 95064

2 Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad

Branch Road, NW, Washington, DC 20015-1305

3 Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii-Manoa,

Honolulu, HI 96822

4 Tennessee State University, Center of Excellence in Information Systems, 3500 John A. Merritt Blvd., Box 9501, Nashville, TN. 37209-1561 - 2 -

ABSTRACT

We present 11 years of HIRES precision radial velocities (RV) of thenearby M3V star Gliese 581, combining our data set of 122 precision RVs with an ex- isting published 4.3-year set of 119 HARPS precision RVs. The velocityset now indicates 6 companions in Keplerian motion around this star. Differential photometry indicates a likely stellar rotation period of≂94 days and reveals no significant periodic variability at any of the Keplerian periods, supporting planetary orbital motion as the cause of all the radial velocity variations. The combined data set strongly confirms the 5.37-day, 12.9-day, 3.15-day, and 67-day planets previously announced by Bonfils et al. (2005), Udry et al. (2007), and Mayor et al. (2009). The observations also indicate a 5th planet in the system, GJ 581f, a minimum-mass 7.0M?planet orbiting in a 0.758 AU orbit of period

433 days and a 6th planet, GJ 581g, a minimum-mass 3.1M?planet orbiting at

0.146 AU with a period of 36.6 days. The estimated equilibrium temperature of

GJ 581g is 228 K, placing it squarely in the middle of the habitable zone ofthe star and offering a very compelling case for a potentially habitable planet around a very nearby star. That a system harboring a potentially habitableplanet has been found this nearby, and this soon in the relatively early history of precision RV surveys, indicates thatη?, the fraction of stars with potentially habitable planets, is likely to be substantial. This detection, coupled with statistics of the incompleteness of present-day precision RV surveys for volume-limited samples of stars in the immediate solar neighborhood suggests thatη?could well be on the order of a few tens of percent. If the local stellar neighborhood is a repre- sentative sample of the galaxy as a whole, our Milky Way could be teeming with potentially habitable planets. - 3 - Subject headings:stars: individual: GJ 581 HIP 74995 - stars: planetary systems - astrobiology - 4 -

1. Introduction

There are now nearly 500 known extrasolar planets, and discoverywork continues apace on many fronts: by radial velocities (RV), gravitational microlensing, transit surveys, coronography, nulling interferometry, and astrometry. By far the most productive discovery technique to date has been through the use of precision RVs to sense the barycentric reflex velocity of the host star induced by unseen orbiting planets. In recent years, the world"s leading RV groups have improved precision down to the≂1 ms-1level, and even below, extending detection levels into the range of planets with masses lessthan 10M?, commonly referred to as "Super-Earths". This level of precision is now bringing within reach one of the holy grails of exoplanet research, the detection of≂Earth-size planets orbiting in the habitable zones (HZ) of stars. Nearby K and M dwarfs offer the best possibility of such detections, as their HZ"s are closer in, with HZ orbital periods in the range of weeks to months rather than years. These low mass stars also undergo larger reflex velocities for a given planet mass. To this end, we have had a target list of≂400 nearby quiet K and M dwarfs under precision RV survey with HIRES at Keck for the past decade. One of these targets, the nearby M3V star GJ 581 (HIP 74995), has received considerable attention in recent years following the announcementby Bonfils et al. (2005), hereafter Bonfils05, of a 5.37-day hot-Neptune (GJ 581b, or simply planet-b) around this star. More recently, the Geneva group (Udry et al. 2007), hereafter Udry07, announced the detection of two additional planets (c and -d) in this system, one close to the inner edge of the HZ of this star and the other close to the outer edge. Planet-cwas reported to have a period of 12.931 days andmsini= 5.06M?whereas planet-d was reported to have a period of 83.4 days andmsini= 8.3M?. The Geneva group"s announcement of planet-c generated considerable excitement because of its small minimum mass (5M?, well below the masses of the ice giants of - 5 - our solar system and potentially in the regime of rocky planets or Super-Earths) and its location near the inner edge of the HZ of this star. An assumed Bond albedo of

0.5 yielded a simple estimate of≂320 K for the equilibrium temperature of the planet,

suggesting the possibility that it was a habitable Super-Earth. However, a more detailed analysis by Selsis et al. (2007), that included the greenhouse effectand the spectral energy distribution of GJ 581, concluded that planet-c"s surface temperature is much higher than the equilibrium temperature calculated by Udry07 and that it is unlikely to host liquid water on its surface. Selsis et al. (2007) concluded that both planets c and d are demonstrably outside the conservative HZ of this star, but that given a large atmosphere, planet-d could harbor surface liquid water. Chylek & Perez (2007) reached a similar conclusion that neither planets c nor d is in the HZ, but that planet-dcould achieve habitability provided a greenhouse effect of 100 K developed. Moreover, if these planets are tidally spin-synchronized, planet-c could conceivably have atmospheric circulation patterns that might support conditions of habitability. von Bloh et al. (2007) also concluded that planet-c is too close to the star for habitability. They argue, however, that if planet-d has a thick atmosphere and is tidally locked, it may lie just within the outer edge of the HZ. Both von Bloh et al. (2007) and Selsis et al. (2007) conclude that planet-d would be an interesting target for the planned TPF/Darwin missions. Beust et al. (2008) studied the dynamical stability and evolution of the GJ 581 system using the orbital elements of Udry07, which they integrated forward for 108years. They observed bounded chaos (see e.g. Laskar (1997)), with small-amplitude eccentricity variations and stable semi-major axes. Their conclusions were unaffected by the presence of any as-yet-undetected outer planets. On dynamical stability grounds, they were able to Last year, Mayor et al. (2009), hereafter Mayor09, published a velocity update wherein - 6 - they revised their previous claim of an 8M?planet orbiting with an 83-day period, to a

7.1M?planet orbiting at 67-days, citing confusion with aliasing for the former incorrect

period. Mayor09 also reported another planet in the system at 3.148 days with a minimum mass of 1.9M?. They also presented a dynamical stability analysis of the system. In particular, the addition of the 3.15d planet, GJ 581e, greatly strengthened the inclination limit for the system. The planet was quickly ejected for system inclinations less than 40
◦. This dynamical stability constraint implies an upper limit of 1.6 to the 1/sini correction factor for any planet"s minimum mass (assuming coplanarorbits). Most recently, Dawson and Fabrycky (2010) published a detailed study of the effects of aliasing on the GJ 581 data set of Mayor09. They concluded that the 67-day period of GJ 581c remains ambiguous, and favored a period of 1.0125 days that produced aliases at both 67 days and

83 days.

The Gliese 581 system exerts an outsize fascination when comparedto many of the other exoplanetary systems that have been discovered to date.The interest stems from the fact that two of its planets lie tantalizingly close to the expectedthreshold for stable, habitable environments, one near the cool edge, and one near thehot edge. We have had GJ 581 under survey at Keck Observatory for over a decade now.In this paper, we bring 11 years of HIRES precision RV data to bear on this nearby exoplanet system. Our new data set of 122 velocities, when combined with the previously published 119HARPS velocities, effectively doubles the amount of RVs available for this star, and almost triples the time base of those velocities from 4.3 years to 11 years. We analyze the combined precision RV data set and discuss the remarkable planetary system that they reveal. - 7 -

2. Radial Velocity Observations

The RVs presented herein were obtained with the HIRES spectrometer (Vogt et al.

1994) of the Keck I telescope. Typical exposure times on GJ 581 were 600 seconds, yielding

a typical S/N ratio per pixel of 140. Doppler shifts are measured byplacing an Iodine absorption cell just ahead of the spectrometer slit in the converging f/15 beam from the telescope. This gaseous absorption cell superimposes a rich forest of Iodine lines on the stellar spectrum, providing a wavelength calibration and proxy for the point spread function (PSF) of the spectrometer. The Iodine cell is sealed and temperature-controlled to 50 ±0.1 C such that the column density of Iodine remains constant (Butler et al. 1996). For the Keck planet search program, we operate the HIRES spectrometer at a spectral resolving power R≈70,000 and wavelength range of 3700-8000°A, though only the region

5000-6200

A (with Iodine lines) is used in the present Doppler analysis. Doppler shifts from the spectra are determined with the spectral synthesis technique described by Butler et al. (1996). The Iodine region is divided into≂700 chunks of 2°A each. Each chunk produces an independent measure of the wavelength, PSF, and Doppler shift. The final measured velocity is the weighted mean of the velocities of the individual chunks. In August 2004, we upgraded the focal plane of HIRES to a 3-chip CCD mosaic of flatter and more modern MIT-Lincoln Labs CCD"s. No zero point shiftin our RV pipeline was incurred from the detector upgrade. Rather, the new CCD mosaic eliminated a host of photometric problems with the previous Tek2048 CCD (non-flat focal plane, non-linearity of CTE, charge diffusion in the silicon substrate, overly-large pixels,and others). The deleterious effects of all these shortcomings can be readily seen aslarger uncertainties on the pre-August 2004 velocities. In early 2009, we submitted a paper containing our RVs up to that date for GJ 581 that disputed the 83-day planet claim of Mayor09. One of the referees (from the HARPS - 8 - team) kindly raised the concern (based partly on our larger value for apparent stellar jitter) that we may have some residual systematics that could be affectingthe reliability of some of our conclusions. In the precision RV field there are no suitable standards by which teams can evaluate their performance and noise levels; so, it is rarebut also extremely useful for teams to be able to check each other using overlapping target stars, like GJ 581, for inter-comparison. So, we took the HARPS team"s concerns to heart and withdrew our paper to gather another season of data, to do a detailed reanalysis of our uncertainty estimates, and to scrutinize our 15-year 1500-star data base for evidence of undiscovered systematic errors. Soon after we withdrew our 2009 paper, Mayor09 published a revised model wherein they altered their 83-day planet period to 66.8 days (citing confusion by yearly aliases) and also announced an additional planet in the system near 3.15 days. For our part, as a result of our previous year"s introspection, we discovered that the process by which we derive our stellar template spectra was introducing a small component of additional uncertainty that added about 17% to our mean internal uncertainties. This additional noise source stems from the deconvolution process involved in deriving stellar template spectra. This process works quite well for G and K stars, but it is prone to extra noise when applied to heavily line-blanketed M dwarf spectra. We have included this in ourpresent reported uncertainties for GJ 581, and are working on improvements to the template deconvolution process. Furthermore, our existing template for this star, taken many years ago, was not up to the task of modeling RV variation amplitudes down in the few ms -1regime. So, over the past year, we obtained a much higher quality template for GJ 581. The HIRES velocities of GJ 581 are presented in Table 1, corrected to the solar system barycenter. Table 1 lists the JD of observation center, the RV, and the internal uncertainty. The reported uncertainties reflect only one term in the overall error budget, and result - 9 - from a host of systematic errors from characterizing and determining the PSF, detector imperfections, optical aberrations, effects of under-sampling the Iodine lines, etc. Two additional major sources of error are photon statistics and stellar jitter. The former is already included in our Table 1 uncertainties. The latter varies widely from star to star, and can be mitigated to some degree by selecting magnetically-inactive older stars and by time-averaging over the star"s unresolved low-degree surface p-modes. The best measure of overall precision for any given star is simply to monitor an ensemble ofplanet-free stars of similar spectral type, chromospheric activity, and apparent magnitude, observed at similar cadence and over a similar time base. Figures 2, 3, and 4 of Butler et al. (2008) show 12 M dwarfs with B-V, V magnitude, and chromospheric activity similar to GJ 581. In any such ensemble, it is difficult to know how much of the root-mean-square (RMS) of the RVs is due to as-yet-undiscovered planets and to stellar jitter. However, these stars do establish that our decade-long precision is better than 3 ms -1for M dwarfs brighter than V=11, including contributions from stellar jitter, photon statistics, undiscovered planets, and systematic errors.

3. Properties of GJ 581

The basic properties of GJ 581 were presented by Bonfils05 and Udry07 and will, for the most part, simply be adopted here. Briefly recapping from Bonfils05 and Udry07, GJ 581 is an M3V dwarf with a parallax of 159.52±2.27 mas (distance of 6.27 pc) with V = 10.55±0.01 and B-V = 1.60. The parallax and photometry yield absolute magnitudes of M V= 11.56±0.03 and MK= 6.86±0.04. The V-band bolometric correction of 2.08 (Delfosse et al. 1998) yields a luminosity of 0.013L?. The K-band mass-luminosity relation of Delfosse et al. (2000) indicates a mass of0.31±0.02M?, and the mass-radius relations of Chabrier & Baraffe (2000) yield a radius of 0.29R?. - 10 - Bean et al. (2006) report the [Fe/H] of GJ 581 to be -0.33, while Bonfils05 report [Fe/H] = -0.25. Both results are consistent with the star being slightly metal-poor, in marked contrast to most planet-bearing stars that are of super-solar metallicity. Johnson & Apps (2009) presented a broadband (V-K) photometric metallicity calibration for M dwarfs that, in conjunction with the star"s broadband magnitudes implies a metallicity of [Fe/H] = -0.049. Most recently, Rojas-Ayala et al. (2010) estimated the metallicity at -0.02, while Schlaufman and Laughlin (2010) cite a metallicity of -0.22. Thus, GJ 581 appears to be basically of solar or slightly sub-solar metallicity, yet has produced at least 4 or more low-mass planets. However, this is no cause for surprise. Laughlin et al. (2004) and Ida & Lin (2005) have argued that the formation of low-mass planets should not be unduly affected by modestly subsolar metallicity. Udry07 report GJ 581 to be one of the least active stars on the HARPS M-dwarf survey, with Bonfils05 reporting line bisector shapes stable down totheir measurement quite inactive with an age of at least 2 Gyr. Our measurement of logR?hk=-5.39 leads to an estimate (Wright 2005) of 1.9 ms -1for the expected RV jitter due to stellar surface activity and an age estimate of 4.3 Gyr.

4. Photometric Observations

Precise photometric observations of planetary host candidate stars are useful to look for short-term, low-amplitude brightness variability due to rotational modulation in the visibility of starspots and plages (see, e.g., Henry, Fekel, & Hall 1995). Long-term brightness monitoring of these stars enabled by our automatic telescopes candetect brightness changes due to the growth and decay of individual active regions as well as brightness variations associated with stellar magnetic cycles (Henry 1999; Lockwood et al. 2007; Hall et al. 2009). - 11 - Therefore, photometric observations of planetary candidate stars help to determine whether the observed radial velocity variations are caused by stellar activity (spots and plages) or reflex motion due to the presence of orbiting companions. Quelozet al. (2001) and Paulson et al. (2004) have documented several examples of solar-type stars whose periodic radial velocity variations were caused by stellar activity. GJ 581 has also been classified as the variable star HO Librae, thoughWeis (1994) reported its short-term variability to be at most 0.006 magnitudes.Udry07 report the star to be constant to within the 5 millimag Geneva photometry catalog precision of V=10.5 stars. We acquired new photometric observations of GJ 581 in the JohnsonV band during the

2007 and 2008 observing seasons with an automated 0.36 m Schmidt-Cassegrain telescope

coupled to an SBIG ST-1001E CCD camera. This Tennessee State University telescope was mounted on the roof of Vanderbilt University"s Dyer Observatory inNashville, Tennessee. Differential magnitudes were computed from each CCD image as the difference in brightness between GJ 581 and the mean of four constant comparison stars in the same field. A mean differential magnitude was computed from usually ten consecutive CCD frames. Outliers from each group of ten images were removed based on a 3σtest. If three or more outliers were filtered from any group of ten frames (usually the result of non-photometric conditions), the entire group was discarded. One or two mean differential magnitudes were acquired each clear night; our final data set consists of 203 mean differential magnitudes spanning 530 nights. Our 203 photometric observations are plotted in the top panel of Figure 1; they scatter about their mean with a standard deviation of 0.0049 mag. Aperiodogram of the observations, based on least-squares sine fits, is shown in the second panel, resulting in a best-fit period of 94.2±1.0 days. That rotation period is quite similar to the rotational - 12 - period of another important M dwarf planet host, GJ 876, and givesadded confidence to the current findings. It is also consistent with GJ 581"s low activity and age estimate. In the third panel, we plot the observations modulo the 94.2-day photometric period, which we take to be the star"s rotation period. A least-squares sine fit onthe rotation period gives a semi-amplitude of 0.0030±0.0004 mag. The window function for the rotation period is plotted in the bottom panel. Five of the six radial velocity periods discussed below are indicted by vertical dotted lines in the second and fourth panels; our data set is not long enough to address the 433-day period of GJ 581f. As will be shown below, none of the five periods coincide with any significant dip in the periodogram.

5. Orbital Analysis

We obtained 122 RVs with the HIRES spectrometer at Keck. The data set spans

10.95 years with a peak-to-peak amplitude of 37.62 ms

-1, an RMS velocity scatter of 9.41 ms -1, and a mean internal uncertainty of 1.70 ms-1. Figure 2 (top panel) presents the RVs tabulated in Table 1, combined with the HARPS RVs published by Mayor09. The 122 (red) hexagon points are the HIRES observations, while the HARPS observations are shown as (blue) triangle points. A zero-point offset of 1.31 ms -1was removed between the two data sets, and Figure 2 has this offset included. The HARPS data consist of 119 observations at a reported median uncertainty of 1.10 ms -1and extending over 4.3 years. The peak-to-peak amplitude of the HARPS data set is 39.96 ms -1. The combined data set has 241 velocities, with a median uncertainty of 1.30 ms -1. For the orbital fits, we used the SYSTEMIC Console (Meschiari et al. 2009; Meschiari & Laughlin 2010). We assume coplanar orbits withi= 90◦and Ω = 0◦. Uncertainties are based on 1000 bootstrap trials. We take the standard deviations of the fitted parameters to the bootstrapped RVs as the uncertaintiesin the fitted parameters. - 13 - Fig. 1.- (Top): PhotometricV-band observations of GJ 581 acquired during the 2007 and

2008 observing seasons with an automated 0.36 m imaging telescope.(Second Panel):

Periodogram analysis of the observations gives the star"s rotationperiod of 94.2 days. (Third Panel): The photometric observations phased with the 94.2-day period reveal the effect of rotational modulation in the visibility of photospheric starspots on the brightness of GJ 581. (Bottom): Window function of the 94.2-day rotation period. The radial velocity periods of 5 of the 6 planetary companions are indicated by verticaldotted lines in the second and fourth panels. - 14 - Fig. 2.- Top panel: Combined RV data of GJ 581 from HIRES (red hexagons) and HARPS (blue triangles). Lower panel: spectral window - 15 - The fitted mean anomalies are reported at epoch JD 2451409.762. The assumed mass of the central star is 0.31M?.For all fits presented here, we fixed the eccentricities at zero since the amplitudes are all quite small and extensive modeling revealed that allowing eccentricities to float for any or all of the 6 planets does not significantly improve the overall fit. The power spectrum of the sampling window is shown in the lower panelof Figure 2. As expected, there is some spurious power created by the samplingtimes near periods of

1.003d (the solar day in sidereal day units), 29.5d (the lunar synodicmonth), 180d (≂1/2

year), and 364d (≂1 year), all artifacts of the nightly, monthly, and yearly periods on telescope scheduling. The top panel of Figure 3 shows the power spectrum of the RV data. Following Gilliland & Baliunas (1987) (hereafter GB87), in Figure 3, we use an error-weighted version of the Lomb-Scargle periodogram. The horizontal lines in the periodograms in Figure 3 roughly indicate the 0.1%, 1.0%, and 10.0% False Alarm Probability (FAP)levels from top to bottom. To determine better estimates of the FAPs of the prominent peaks in the periodograms, we define the noise-weighted power in a prominent peak with (GB87) p 0=N 4x 2

0σ20,(1)

where N is the number of observations,x0is the RV half-amplitude implied by the peak, andσ20is the variance in the data or residuals prior to fitting out the implied planet. Additionally, we can also define power in a prominent peak as (Cumming (2004)): p

0=(N-2)

2(χ2constant-χ2circ)χ2circ,(2)

whereχ2circis the reduced chi-squared for a circular fit at/near the period implied by the peak andχ2constantis the reduced chi-squared for a constant RV model of the data or residuals. Estimation of the false-alarm probability of a given peak requires knowledge of - 16 -quotesdbs_dbs43.pdfusesText_43

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